Our focus during the last funding period was on molecular mechanisms driving androgen receptor (AR) activity in castration resistant prostate cancer (CRPC) and on the specific functions of AR in CRPC. We have shown that increased intratumoral androgen synthesis contributes to the reactivation of AR transcriptional activity in CRPC. This mechanism is targeted by CYP17A1 inhibitors, including the recently FDA approved drug abiraterone. Unfortunately, most patients who respond to abiraterone relapse within 1-2 years, and current data indicate that AR is active in these recurrent tumors. Additional mechanisms that enhance AR activity and drive tumor progression in CRPC are almost certainly also contributing to abiraterone resistance/relapse. Moreover, while TMPRSS2:ERG fusion and PTEN are examples of frequent genomic alterations in prostate cancer, it is becoming clear that multiple additional genomic events may be contributing in small subsets of patients, and that many of these may have therapeutic implications. This may be particularly true in advanced CRPC, and we hypothesize that a diversity of genomic alterations may be contributing to AR activation and tumor progression in CRPC and abiraterone-resistant CRPC. Our objective in Aim 1 is to identify mechanisms mediating abiraterone-resistant CRPC. During the last funding period we also demonstrated epigenetic reprogramming of AR function in CRPC. In particular, we found that AR in CRPC cells is recruited to and stimulates the expression of genes that drive cells through mitosis. Previous studies have shown that the epigenetic regulator EZH2 is up-regulated in CRPC. In recent studies we have found that EZH2 can be recruited to the cis-regulatory elements of mitotic genes targeted by AR in CRPC. Significantly, EZH2 directly up-regulates these AR targets in CRPC cells and stimulates their growth. This novel EZH2 function is dependent on its methyltransferase activity, but is not dependent on H3K27 methylation. Finally, we have found that phosphorylation of EZH2 by CDK1 alters its activity. These findings, in conjunction with our data showing that AR is also phoshorylated and activated by CDK1, indicate that a positive loop involving AR, EZH2, and CDK1 may be driving AR activity and proliferation in a subset of CRPC. Aim 2 will test the hypothesis that EZH2 reprograms AR function in subsets of CRPC patients, and Aim 3 will assess in preclinical models and a clinical trial the efficacy of agents that suppress AR and EZH2 function through inhibiting CDK1.